In the Yvan J. Beliveau et al. invention ( U.S. patent application Ser. No. 07/570,268), a fixed referent station is used to produce a laser beam which is rotated at a constant angular velocity. When the rotating laser beam crosses a unique point in its rotation, this point being known as the rotation datum, a secondary transmitter such as a strobe transmitter is triggered. Alternatively, the secondary transmitter can be a radio transmitter. When triggered, the strobe transmitter broadcasts a signal which is received at the portable position sensor. The received signal is then translated into a time datum corresponding to the time which the rotating laser beam crossed its rotation datum. This time datum is the reference by which other time data are compared, in order to calculate horizontal and vertical angles of intersection.
The use of a secondary transmission system which is separate and unique from the rotating laser beam, as in the Beliveau et al. application, has certain disadvantages. First, a separate transmission system increases the complexity of the fixed referent station system which increases the cost and reduces the reliability of the positioning system. Second, if a radio transmitter is used as the secondary transmission source, a radio receiving system is required at the portable position sensor, which further increases the cost and reduces the reliability of the positioning system. Third, a sensor is required at the laser referent station which can be accurately triggered when the laser beam crosses the rotation datum. Each of these active components of the secondary transmission system contributes error to time measurements used for position calculations. The present invention eliminates the need for a secondary transmission system.
Additionally, as described in the Yvan J. Beliveau et al. application, the portable position sensor includes a laser detector having a flat disk with a photosensitive material on its outside circumference. A variation of this detector described in the Beliveau et al. application is a multi-faceted polygon having photosensitive material located on each facet. The present invention includes a simplified detector having a 360 degree lens and one small photodetector.